1. Technical Field of the Invention
This invention relates to a method of and apparatus for indicating battery capacity in which the remaining capacity of a battery during practical operation is indicated by a meter or is displayed in a digital mode by using a liquid crystal display unit or an LED display unit, and more particularly to a method of and apparatus for indicating battery capacity in which the remaining capacity of a battery during practical operation can be indicated not only when the battery is not discharged but also when it is being discharged.
2. Description of the Prior Art
In the most popular method of monitoring the battery capacity, the specific gravity of the electrolyte is measured to estimate the battery capacity. At the present time, this method is most accurate.
However, an excellent technique of converting the specific gravity of the electrolyte into an electrical signal is not provided yet. When the specific gravity is detected according to the above-described method, the sensor is usually inserted in the pilot cell only, and therefore it is difficult to detect accurately the condition of the battery other than that of the pilot cell. Furthermore, since the rate of decrease of the specific gravity during discharge depends on the type of battery and since the diffusion speed of the electrolyte in the battery depends on the configuration of a battery, the measured specific gravity must be adjusted according to battery type and battery configuration. Practically, it is too difficult to adjust them. In addition, in the case of fork lift use in which the battery is detachably mounted, the maintenance of the sensor circuit is troublesome, especially when the battery is exchanged, and often the sensor circuit becomes defective. Accordingly, for a movable battery such as a fork lift battery, the type of a remaining capacity monitor in which the sensor is inserted in a battery electrolyte to measure the specific gravity, is scarcely used. It is partially mounted only for some stationary batteries.
The capacity of an electric vehicle battery may be determined by using a voltmeter connected to the terminals thereof However, the voltmeter cannot determine the remaining capacity of the electric vehicle battery. In this case, the capacity of the battery is determined from the minimum value which is indicated by the voltmeter when the battery is under load. In this method, the voltmeter used is a simple instrument and the indication of the capacity by voltmeter is relatively accurate. However, since the indication is always variable, it is difficult to determine the capacity with high accuracy for the operator until he becomes familiar with the behavior of the voltmeter.
In order to overcome this difficulty, a voltmeter for monitoring the battery capacity has been developed in which a capacitor is installed to delay the response of the meter thereby to prevent the abrupt operation of the pointer of the voltmeter. In addition, a method has been disclosed in which whenever the voltage of the battery is decreased sequentially to several particular preset values, the pointer of the voltmeter is electrically locked, so that the pointer is deflected stepwise as the capacity decreases. In these methods, the indication is variable or discontinuous, and the voltmeter, unlike a fuel gauge in an automobile, cannot continuously indicate the decrease of capacity during discharge. In order to eliminate this drawback, a system has been proposed, for instance, by U.S. Pat. No. 4,180,770 in which the open-circuit voltage of a battery is adjusted according to a voltage drop which occurs when the battery is discharged, and the open-circuit voltage thus adjusted is utilized to indicate the battery capacity thereof. Namely, the open-circuit voltage in this system indicates the remaining capacity of a battery which is able to discharge still more. Therefore, a capacity meter according to this system currently may be the most practical. However, this system is still disadvantageous in the following points: Since the battery capacity generally depends on the load profile applied to the battery, it is necessary to adjust the open-circuit voltage of the battery according to the load profile applied thereto. In addition, in the case when the battery has been replaced by another one, the capacity is not indicated unless a load is connected to the battery, even if the capacity of the battery has been reduced.
A remaining capacity meter based on two factors, load current and voltage drop, was used before. However, the adjustment of the meter includes the control of two variables, is very difficult. It is necessary to use a shunt resistor for current detection, and the detecting circuit is intricate.